Consistent with requirements for increased productivity, off-highway vehicles such as large agricultural tractors have steadily increased in size with the introduction of new models. In some models, such size increases were accompanied by diminished vehicle maneuverability because only the front wheels were steerable. More recently, improved vehicle designs provide for steering of all wheels. In response to such designs, improved steering systems have been developed so that as the front wheels are steered by the operator, the rear wheels are automatically steered in response thereto.
While earlier automatic steering systems were generally satisfactory, they were not without certain deficiencies. An example of such an earlier system is shown and described in U.S. Pat. No. 4,175,638 (Christensen) which is assigned to the same assignee as this invention. The Christensen system is of the analog type; that is, proportional voltages are used for several functions such as hydraulic steering valve control. For a given "standard" of accuracy, reliability and position resolution, analog systems are generally more expensive and difficult to maintain at optimum performance. This is particularly true where the system is mounted on a hard-worked, sometimes-abused off-highway vehicle.
Such system employs potentiometers to obtain the feedback signal and the setting of such potentiometers is quite critical. Potentiometer setting is required to be done with precision and occasions the use of a volt-ohmmeter, an instrument sometimes not quickly available in the field--even if there are personnel at the site who know how to use it.
Such system uses a control valve to direct hydraulic oil to the wheel steering cylinder(s) at a rate required to maintain the automatically-steered rear wheels in position coordination with the front, manually-steered wheels. Such valves have a minimum or "threshold" input power at which wheel movement is produced. If the valve is operated below this input power level, no wheel steering motion results. If, like the Christensen system, the minimum input power level is recognized in the control protocol, such power level is required to be determined by a technician and "set" in the controller to achieve satisfactory system operation. Such determination and setting is time-consuming and if the control valve is required to be replaced for some reason, is often required to be done anew. No two control valves have precisely the same threshold input power. If the system is of the type where threshold input power is not recognized and set, the system tends to be sluggish and less responsive.
Another disadvantage of such system is that even after setting the threshold input power, system responsiveness may suffer over time due to changes in the temperature of the hydraulic valve coil or coils. Such changes in temperature are usually accompanied by changes in coil resistance and therefore, with a constant input voltage, in coil current. Changes in such current affect valve and system performance.
Still another occasional disadvantage of such system (and, probably, others of its type) is that the automatically-steered wheels, if "locked" in the centered position while steering with only the front wheels, tend to drift out of position to an unacceptable angle. As a result, tires are sometimes scuffed and wear prematurely and the vehicle becomes less fuel-efficient, albiet perhaps only incrementally so. Wheel misalignment also causes unnecessary stress on steering and suspension components.
Yet another disadvantage of such earlier system and others of that type is that diagnosis of component failures is quite difficult. Often, the vehicle operator is unable to explain the resulting steering impairment with certainty. In the case of an intermittent "error" or failure, the servicing technician is occasionally required to spend a great deal of time riding and/or watching the vehicle in order to witness such failure so that necessary repairs can be performed.